The present invention relates generally to vehicular rear-wheel steering control. More particularly, the invention relates to enhanced transient response for vehicles with active rear steering.
For vehicles having four-wheel steering systems, active rear steering may improve maneuverability. For example, active rear steering may reduce the turning radius and enhance vehicle handling by improving both quickness of response and stability. At present, vehicle steering control algorithms may include a feed-forward static gain and a feedback yaw control. The feed-forward gain is selected to reduce turning radius at low speeds and to increase vehicle stability at high speeds. The feedback yaw control is primarily designed to improve system robustness and vehicle stability at or close to the limit of adhesion.
In active rear steering, the vehicle rear wheels may be steered at an angle proportional to that of front wheels. The gain relating the rear and front steering angles is a function of speed, being negative at low speeds and positive at high speeds. At low speeds, the rear wheels are steered in the direction opposite to the front wheels thereby improving maneuverability (i.e., tightening the radius of vehicle path). At high speeds, the rear wheels are steered in the same direction as the front wheels to promote stability. In order to further enhance vehicle handling and stability, a closed loop yaw control algorithm may be added to the existing feed-forward control. The closed loop yaw control algorithm may be derived from a brake based stability enhancement algorithm.
To provide desirable handling characteristics, phase lags in yaw rate phase lags and lateral acceleration responses to steering angle should be short and remain relatively consistent with one another throughout the range of speeds. Quick response to steering input may prevent the driver from oversteering. In addition, consistent transient behavior may allow the driver to more properly anticipate vehicle response.
The feed-forward static gain and closed loop yaw control algorithm may have limitations in improving vehicle handling within the linear range. For example, when the rear wheels are steered in proportion to the front wheels (e.g., a static feed-forward gain), the lag in yaw rate transient response may not be significantly reduced as compared to a front steer vehicle. Furthermore, the closed loop yaw control algorithm may not generate a significant correction for when vehicle is in the linear range of handling, even if control xe2x80x9cdead-bandxe2x80x9d is removed.
One way to overcome the problem is to use the desired yaw rate derived from a static relationship between yaw rate, steering angle and speed, rather than using a yaw rate estimated from vehicle dynamics. In this way, the desired yaw rate may precede the actual yaw rate during quick transient maneuvers. As such, a closed loop steering correction may be produced thereby reducing vehicle response lag. This advantage may be provided if additional accurate sensors permitting closed loop control (such as yaw rate sensor and lateral acceleration sensor) are available. The cost of sensors, especially the yaw rate sensor may increase the overall cost of the system. Accordingly, it would be desirable to overcome the problem without the need for additional sensors.
Therefore, it would be desirable to provide a method for transient response enhancement for vehicles with active rear steering that overcomes the aforementioned and other disadvantages.
One aspect of the present invention provides a method for steering a vehicle having driver-controlled front wheel steering and active rear wheel steering. A rear wheel steering angle static portion is determined based on a control gain and a front wheel steering angle. A slew-limited magnitude of desired lateral acceleration is determined based on a desired lateral acceleration. A multiplying factor is determined based on the slew-limited magnitude of desired lateral acceleration. A feed-forward rear steering angle is determined based on the multiplying factor, the rear wheel steering angle static portion, and a rear wheel steering angle dynamic portion. The active rear wheel steering may be controlled electronically. The multiplying factor may equal one when the slew-limited magnitude of desired lateral acceleration is less than an assigned value of about 8 m/s2. The multiplying factor may equal zero when the slew-limited magnitude of desired lateral acceleration is greater than an assigned value of about 12 m/s2. The multiplying factor may be determined based on a magnitude of yaw rate error and/or a linear interpolation. The feed-forward rear steering angle may be determined in accordance with a Laplace domain expression and/or based on at least one value table. The rear wheel static feed-forward gain, Kff, the lead gain, K1, and the time constant, T, may be functions of vehicle speed
Another aspect of the invention provides a computer usable medium, including a program, for steering a vehicle having driver-controlled front wheel steering and active rear wheel steering. The invention provides computer readable program code for performing the method steps described above.